1. Field of the Invention
The present invention is a dual frequency, transmit-receive module for use in any radar system. This dual frequency transmit-receive module enables the radar system to transmit or receive a radio frequency signal of an original, predetermined, frequency f.sub.o and a second harmonic signal 2f.sub.o, that is two times that of the original frequency. This novel transmit-receive module although operable in any radar system, has particular applicability to active aperture radar systems, wherein one antenna is used for each transmit-receive module signal transmission and/or reception.
2. Description of the Prior Art
The present invention relates generally to the field of radio frequency transmitting and receiving systems and more particularly to systems which, such as active aperture radar systems utilize a common antenna for each transmit-receive module.
A radar system in its simplest configuration is a remote location, radio frequency transmission and receiving means used to ascertain the distance and size of targets located in the environment outside of the radar system.
The target acquisition system operates through the transmission and return reflection of, a radio frequency signal of predetermined frequency. This amplified, pulsed, signal of predetermined frequency is first; generated by a frequency generator, driven by a pulsed modulator and amplified by an amplifier where it then transmitted to a single, mechanically steered antenna by means of a duplexer. The radar system duplexer serves to maintain signal separation between transmitted signals of a predetermined frequency and the received reflected signals which bounce off of the targets and return to the single, mechanically steered, physically sweeping antenna. The received reflected signals first enter the "front end", or low noise amplifier, which determines the signal sensitivity. These received target reflected signals finally reach a signal processor, wherein the distance and the size of the target reflecting the transmitted signal f.sub.o is ascertained.
The above-described radar system is well known in the prior art and has been traditionally designed in either the broadband or narrow band frequency range.
An important variation of this standard radar system and also well known in the art is the electronically scanned antenna radar system which utilizes an array of individual antenna elements, whose phase is controlled so that the beam formed by the array is electronically steered.
In such an electronically scanned radar system, the composite array, composed of many antenna elements, is no longer physically, mechanically steered or scanned from one position to another. The electronically steered system contains a manifold which is operable to split the single signal of predetermined frequency into a multiplicity of signals. Each one of these individual unaltered signals is then phase shifted through individual phase shifters a specified, predetermined phase difference. These individual phase shifters then transmit the shifted signals through individual, arrayed antennas, wherein each antenna element transmits uniquely each phase shifted signal. The reflected, received signals from various targets, again of the same predetermined transmitted frequency return via a receiving manifold into a common receiver.
An active aperture radar system in its most elementary form comprises; a signal generator operable to produce a signal of predetermined frequency, f.sub.0, a manifold, operable to split this single signal f.sub.o into a multiplicity of signal paths while maintaining the predetermined frequency, a transmit-receive module for each signal path and an antenna element which is operable to both transmit or receive the signal f.sub.o for each transmit-receive module.
The received, reflected signal f.sub.o, in the standard active aperture radar system, can never be a harmonic of the original signal f.sub.o. Nor can the prior active aperture radar system transmit a second harmonic 2f.sub.o of signal f.sub.o. The active aperture radar system is a more complex system than the passive, electronically steered radar system. However, the active aperture system has less insertion loss resulting in greater transmitter efficiency and greater receiver sensitivity plus the advantage of multiple redundancy.
If one transmit-receive module, or one antenna of an array of these active aperture elements fails, the entire radar system remains functional. Radar system downtime is therefore greatly reduced. To date, in the design of active aperture radar system transmit-receive modules engineers have been restricted to the design of; either high power, narrow band, transmit-receive modules or, low power wide band transmit-receive modules. Wide-band, low power, designs are inherently inefficient.
The problem to be solved then is the development of a dual band capability for the transmit-receive module of a radar system, specifically an active aperture radar system. The preferred embodiment of this invention would provide dual band capability in the transmission and reception of a radio frequency of a predetermined frequency without the efficiency penalties of a wideband low power design.